We have a septic tank and never put bleach down it.
Recently, the bottom of the toilet has developed a dark brown stain.
In the UK I would just pour in bleach and it would go overnight, but obviously I cannot do that here in France.

I use bleach, and we have a septic tank (well actually we have several but I don't like to boast ). If you look in the supermarkets at the toilet cleaner you can use anything which says "sans danger pour fosse septique" which is most of them. Some of the ones with lots of bleach say not more than twice a week, so I use them on the weekly clean but don't leave them in the gites for the guests.

Other alternatives to remove the brown stains are PH minus (used in swimming pools), vinegar or coke. Don't however use hydrocloric acid which is sold for detartering as this will not do your fosse any good._________________www.vendeeblog.net

It was in a book someone (who obviously thought I needed domesticating) bought me once for Christmas.
I actually found some useful tips in it, including one about chucking a can of coke cola down the loo to make it gleam! (works on white sinks too)
And if you ever have a rusty screw you can't remove with the screwdriver...put some cola on it and 5 mins later you'll be able to screw it out